Hot melt adhesives are well known, having been used for decades. Generally, these adhesives comprise thermoplastic materials which when heated melt and are capable of wetting a substrate. When cooled, these materials solidify and firmly adhere to the substrate. Compositions which have been used as hot melt adhesives include polyolefins, such as polyethylene and polypropylene, polyesters such as polyvinyl acetate and polyamides.
Each of these hot melt adhesives possess or exhibit particular characteristics which makes the use of one preferable over another in a particular application. One characteristic which is generally not exhibited by known hot melt adhesives is substantial tensile strength. As a rule, hot melt adhesives are incapable of adding structural support to an article to which they are bonded since they tend to pull apart easily. Furthermore, these hot melt adhesives are generally ineffective when used to bond a series of articles together by laying a strip of the hot melt adhesive around and onto the periphery of the articles as opposed to between the individual articles. Thus, they are basically incapable of bundling and strapping articles together.
A technology not normally related to adhesive bonding is thermotropic mesomorphism. Specifically, this refers to the production of a mesomorphic state in a material by means of heat. Mesomorphic state refers to a molecular state or organization of a composition which is neither a true liquid nor a true crystal. Compounds which enter this mesomorphic state are commonly referred to as liquid crystals.
Two of the types of liquid crystalline materials are nematic liquid crystalline material and smectic liquid crystalline material. A smectic liquid crystalline material can be forced to assume a stratified molecular structure, the molecules being arranged in layers with their axes approximately normal to the plane of the layers. This state does not improve the tensile strength of the smectic liquid crystalline material. A nematic liquid crystalline material can be forced to assume a structure where the molecules assume a parallel or nearly overlapping parallel orientation. The molecules, while in this nematic state, are thread-like or rod-like and can readily be forced or arranged into the parallel, overlapping structure.
The term thermotropic refers to the means by which the liquid crystalline material is forced into the mesomorphic state. A thermotropic liquid crystal is one which is caused to enter a mesomorphic state by heat. This is contrasted by a lyotropic liquid crystal which is caused to enter the mesomorphic state by solvation. For example, the Aramids.TM., which are aromatic polyamide liquid crystals, are caused to go into the nematic state by the application of concentrated sulphuric acid.
Thus, a thermotropic nematic liquid crystalline substance is a substance which when heated, undergoes mesomorphism entering a nematic state. When cooled, the molecules of this material generally retain their organization and thus exhibit synergistic tensile strength.